Abstract- the Space Shuttle Ultra High Frequency (UHF)
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Source of Acquisition NASA Johnson Space Center Shian U. Hwu, Justin A. Dobbins, Yin-Chung Loh, Quin D. Kroll, and Catherine C. Sham Abstract- The Space Shuttle Ultra High Frequency (UHF) communications system is planned to provide Radio Frequency (RF) coverage for astronauts working underside of the Space Shuttle Orbiter (SSO) for thermal tile inspection and repairing. This study is to assess the Space Shuttle UHF communication performance for astronauts in the shadow region without line-of-sight (LOS) to the Space Shuttle and Space Station UHF antennas. To insure the RF coverage performance at anticipated astronaut worksites, the link margin between the UHF antennas and Extravehicular Activity (EVA) Astronauts with significant vehicle structure blockage was analyzed. A series of near-field measurements were performed using the NASNJSC Anechoic Chamber Antenna test facilities. Computational investigations were also performed using the electromagnetic modeling techniques. The computer simulation tool based on the Geometrical Theory of Diffraction (GTD) was used to compute the signal strengths. The signal strength was obtained by computing the reflected and diffracted fields along the propagation paths between the transmitting and receiving antennas. Based on the results obtained in this study, RF coverage for UHF communication links was determined for the anticipated astronaut worksite in the shadow region underneath the Space Shuttle. Shian U. Hw,Justin A. Dobbins, Yin-Chung Loh, Quin D. Kroll, and Catherine C. Sham the Space Shuttle structure obstructions. The Radio Frequency Abstract- The Space Shuttle Ultra High Frequency (UHF) (F@) signals have to overcome the vehicle structure blockage communications system is planned to provide Radio Frequency along the propagation paths. To ensure the RF coverage can 0 coverage for astronauts working underside of the Space meet the requirements, the link margin evaluation must take into Shuttle Orbiter (SSO) for thermal tile inspection and repairing. account such antenna pattern distortion and signal attenuation This study is to assess the Space Shuttle UHF communication performance for astronauts in the shadow region without due to blockage. The system must be able to compensate for the line-of-sight (LOS) to the Space Shuttle and Space Station UHF signal attenuation, and provide adequate signal level throughout antennas. To insure the RF coverage performance at anticipated designated areas of coverage for the communication links to astronaut worksites, the link margin between the UHF antennas function effectively. and Extravehicular Activity (EVA) Astronauts with significant In shadow regions, such non-line-of-sight (NLOS) locations vehicle structure blockage was analyzed. A series of near-field measurements were performed using the NASA/JSC Anechoic will experience relatively high path loss and signal degradation. Chamber Antenna test facilities. Computational investigations The signal degradation and path loss caused by structure were also performed using the electromagnetic modeling blockages are in general much higher for signals in the techniques. The computer simulation tool based on the microwave frequency bands, such as Wireless LAN (2.4 and 5 Geometrical Theory of Diffraction (GTD) was used to compute the GHz), than for those used by the Space Shuttle astronaut signal strengths. The signal strength was obtained by computing communications at the UHF frequency bands (414.2 and 417.1 the reflected and diffracted fields along the propagation paths between the transmitting and receiving antennas. Based on the MHZ). results obtained in this study, RF coverage for UHF communication links was determined for the anticipated astronaut worksite in the shadow region underneath the Space Shuttle. Index Terms-Geometrical theory of diffraction, Space shuttles, Space vehicle communication, UHF communication. I. INTRODUCTION HE Space Shuttle Orbiter (SSO) payload bay Ultra High TFrequency (UHF) antenna is planned to provide UHF communication links for Extravehicular Activity (EVA) astronauts in the worksite underneath the Space Shuttle for thermal tile inspection and repairing, as shown in Figure 1. The payload bay antenna pattern will be blocked and distorted due to Fig. 1. Astronauts in worksite underneath Space Shuttle for thermal tile inspection and repairing. Manuscript received October 14,2004. This work was supported in part by the NASA Johnson Space Center under Contract NAS9-19100. Shian U. Hwu is with Lockheed Martin Space Operations, Houston, Texas A key goal of the work was to avoid the assumption that any 77058 USA (phone: 281-333-7559; fax: 281-333-7620; email: blockages of the line-of-sight 0.0s) automatically precluded [email protected]). Justin A. Dobbins is with NASA Johnson Space Center, Houston, Texas the site from having communication coverage. Instead, This 77058 USA. (e-mail: [email protected]). study is based on a detailed 3D electromagnetic modeling, Yin-Chung Loh is with Lockheed Martin Space Operations, Houston, Texas which took into account difiactions along propagation paths 77058 USA (e-mail: [email protected]). over and around blockage structures. In many cases, as Quin D. Kroll is with NASA Johnson Space Center, Houston, Texas 77058 USA. (e-mail: [email protected]). predicted by the computer simulations and supported by the Catherine C. Sham is with NASA Johnson Space Center, Houston, Texas experimental measurements, the difiactions can result in 77058 USA. (e-mail: [email protected]). adequate received signals and link margin for communication coverage. 0-7803-8812-7/05/$20.00 0 2005 IEEE The purpose of this study is to assess the signal strengths and at the UHF frequency. Thus, the Space Shuttle structures are RF coverage performance for astronauts working in the shadow modeled as perfect electrical conductors. region underneath the Space Shuttle. Both experimental The Space Shuttle payload bay and Space Station UHF measurements and computer simulations were performed. Due antennas are quadrifilar helical antennas. The operating to the large size of the Space Shuttle, full-scale vehicle mockup frequencies are 414.2 MHz and 417.1 MHz. An average measurements requiring a large outdoor range are difficult to frequency of 41 5.65 MHz was used in this analysis for the UHF implement. The scaled Space Shuttle mockup measurements in communications. Anechoic Chamber are feasible. Another approach in evaluating B. Signal Strength Computations the communication performance under vehicle blockage is using an electromagnetic modeling technique [ 1-71. Computer At high frequencies the scattering fields depend on the simulations have advantages when (1) it is too expensive and electrical and geometrical properties of the scatterer in the dangerous to perform real tests or experiments on an existing immediate neighborhood of the point of reflection and complex vehicle; (2) the vehicle is not available or cannot be diffraction. In the field computation, the incident, reflected, and tested for a real scenario. difftacted fields are determined by the field incident on the reflection or difftaction point multiplied by a dyadic reflection Using simulation tools for initial investigation before the or diffraction coefficient, a spreading factor, and a phase term. actual measurements are performed for the final configuration The reflected and diffracted field at a field point r', Fd(r'), in can reduce the project costs and time. A reliable analytical tool general have the following form: provides matrix comparison for proposed approaches and helps to find the best solutions. Modem high-speed computers with Pd(r') = P(r) PdA',~(s) e-jh . (1) large storage capacity have made possible the theoretical calculation of the signal strength with antennas mounted on where E'(r) is the field incident on the reflection or diffraction complex structures such as Space Shuttle. The main advantage point r, D',d is a dyadic reflection or diffraction coefficient, of this mathematical method is that, once the geometry of the kd(s) is a spreading factor, and s is the distance from the vehicle has been represented in the computer, the analyses of reflection or diffraction point r to the field point r'. andkd different astronaut positions can be easily evaluated. The RF can be found from the geometry of the structure at reflection or coverage analysis can be performed on the basis of a computer diffraction point r and the properties of the incident wave there. evaluation; the number of measurements can be cut down to a Thus, the total fields (Et0? can be obtained by summing up the reasonable minimum for verification and validation purpose. individual contributions of the direct field (E?, reflected field (Eref),and diffracted field (Ed1?along the propagation paths, as Since the Space Shuttle is large in terms of wavelength, the following, Geometrical Theory of Diffraction method (GTD) is a suitable candidate for the computational task. The Method of Moments (MOM) is a feasible tool that provides better accuracy. However, excessive computer resources and computing time will be required for electrically large Space Shuttle structure Emt: Total field at the observation point, models. The MOM technique is suitable to verify GTD results E& : Direct fields from antennas, with partial structure models immediate to the antenna. In this Eref: Reflected fields from plates and curve surfaces, study, the GTD was used to compute the electric fields. The E'f : Difftacted fields